The insulation diagnosis based on detecting and interpreting partial discharges is one of the most promising methods for evaluating the insulation condition of high voltage electrical material, such as power transformers, instrument transformers, reactors, switchgear, and particularly of the cables with the accessories thereof installed in power grids.
A “partial discharge” herein refers to an electric discharge affecting a limited part of the insulation where it is produced, without causing its immediate failure but rather its progressive degradation, except in the in case of insulation formed by the ambient air, because degradation due to ionisation is compensated with natural renewal.
However, many of the instruments available on the market include methods with serious limitations when taking on-site measurements in environments characteristic of high voltage installations with a high level of electric noise.
There are three important practical difficulties in the on-site measurements of partial discharges of HV cables which the present invention seeks to solve:                On one hand, the difficulty in distinguishing electric PD signals due to defects of the high voltage insulation of the electric signals characteristic of the electric noise of the environment (radio broadcasters, mobile telephony, white noise, etc.) masking the PD signals;        on the other hand, the difficulty in locating the position of the sources of PDs along the length of the cable in order to make the relevant repair of the faulty area, and        finally, the difficulty in identifying the eventual defects involved in a specific position of the cable (for example in a cable termination) to enable assessing the severity of the defect and acting accordingly. By way of example, it is known that PDs due to the corona effect in the ambient air are not crucial for the insulation failure, whereas PDs due to an internal defect of the cable will inevitably cause the perforation of the insulation sooner or later. It is therefore necessary to know the severity of the sources of PDs.        
Current techniques applied to field PD measurements try to solve some of these problems in a different manner but with serious limitations, as is explained below:
a) Problems with Electric Background Noise:
Most of the known methods try to remove the electric background noise by assuming that it is located in a frequency band in which filtering is performed. It must be pointed out that the very conception of this filtering technique causes the removal or attenuation of the noise along with the attenuation or removal also of the partial discharge signal to be measured for the filtered frequency range.
The frequency spectrum of the noise signal is analysed in other known methods in order to choose a measurement frequency band where the amplitude of the noise is the lowest possible amplitude. The drawback of this method is that sometimes the lowest noise signal band coincides with the band where the PD signal is also weak in amplitude, so the measurement of the PD is poor and inefficient. For example, if the frequency chosen for the measurement is high, then the distance attenuation can be excessive and sensitivity to the partial discharges occurring in positions away from the sensor insufficient.
Finally, another known method tries to remove noise by means of classifying the recorded signals (PDs+noise) into clusters. The clusters are formed by means of determining parameters associated with the signal shape (duration and frequency) and its amplitude. The specific drawback of this method is that the processing is performed by signal level, such that to assure the capture of PD signals the acquisition level must be reduced, and therefore the noise signal content considerably increases. Processing becomes very laborious because the noise is put together with the PDs.
All these indicated methods further have serious limitations concerning white noise, the spectrum of which covers all the frequencies of the PD signal. Frequency filtering techniques cannot be applied because the PD signal would also be lost, and a noiseless frequency band cannot be chosen either because there is a noise signal in all of them and PD clusters having a frequency different from that of the noise cannot be distinguished either.
To remedy the preceding the problems, the present invention proposes performing Wavelet transform of the recorded signal and statistically analysing its components in order to find transient events characteristic of PD pulses which are distinguished from the statistical evolution of the electric noise. The pulses recognised as transient PDs different from the noise can come from insulation failures originated in the cable or in other high voltage equipment or can also come from the power electronics connected to the grid because the power electronics causes transient events similar to the PD pulses characteristic of insulation failures. The identification tool for identifying the type of PD pulses through their patterns as a function of the phase difference with the voltage allows efficiently classifying the clusters of pulses, distinguishing those due to insulation failures from those due to the power electronics.
b) Problem with Locating the Position of the Pulse:
Most of the known methods deal with locating the PD pulses by means of the reflectometry technique which consists of acquiring signals in a measurement station located at one of the ends of the cable and determining the time delay between the signal coming directly from the source of PDs and the signal coming from the reflection at the opposite end of the cable where the circuit is left open. The position of the sources of PDs along the length of the cable is determined by taking into account the propagation speed of the PD signal through the cable. The efficacy of this method is limited for the following reasons:                The reflected signal must travel to the final end of the open cable and return along the entire length of the cable. Accordingly, for long cables having a length exceeding a kilometer, the reflected signal can arrive so attenuated that many PD pulses could not be detected and accordingly their position could not be identified. This problem is accentuated in the diagnosis of dry cables in which small PD amplitudes characterise a high risk of insulation failure, attenuation of the signal being critical for its detection;        The final end of the cable must be open in order to achieve maximum reflection of the signal, which complicates being able to apply this technique when the cable is in service, i.e., connected to the grid.        
Other techniques consider that occurring PDs can only come from accessories (terminations or junctions), so sensors are arranged in each and every one of the accessories in order to attribute the location of the sources of PDs to the accessory where the amplitude of the PD signals is higher. The drawback of this method is based on the simplification of assuming that only accessories are susceptible to insulation failures, overlooking the fact that the cable itself is exposed to manufacturing defects or to assembly or operating damages. This method further requires arranging a sensor in each accessory, which is not always possible, especially in medium voltage grids in which the associated cost of the sensors and the instrumentation in each accessory does not justify applying this diagnostic technique.
Another technique consists of arranging at least two sensors along the length of a cable in different locations. When the PD signal in one of the sensors exceeds a specific level greater than the background noise, the signals coming from at least two different sensors are captured in a synchronised manner. By comparing arrival times of the same signal to the different sensors, the position of the source of PDs is determined. The drawback of this technique consists of the PD pulses having to be greater than the background noise level, which makes it difficult to find PD pulses under the background noise.
In the proposal of the present invention to solve this problem, the reflectometry technique is not used, nor is the PD signal reaching one of the sensors expected to exceed a specific level (background noise) to perform a capture of all the sensors. In the new method of the invention at least two sensors in different locations along the length of the cable are used and periodic captures synchronised per complete periods of the grid voltage wave (for example, a period of the grid voltage wave, 20 ms for 50 Hz and 16.6 ms for 60 Hz, is captured each minute) regardless of the existing background noise are performed. After each capture, the electric background noise is removed. The arrival time delay of the same PD signal to two sensors in different locations allows identifying the position of the source producing the partial discharges. This methodology can only be effectively applied as a result of the effective removal, and in a first instance, of the background noise signals, which allows clearly observing the captured PDs, locating the position of the sources of PDs in extremely severe electric noise environments.
c) Problem with Identifying the Type of Defect Associated with PDs:
The identification of the type of source associated with the measured PDs is not resolved in most diagnostic techniques for diagnosing cable insulation condition, leaving this decision to the operator's judgment. Some techniques use the phase difference of the PDs in relation to the voltage applied for generating a pattern of the sources of PDs in order to help the operator make a decision.
Characteristic patterns of PDs in phase with the voltage, which are referred to as reference patterns of PDs, are known to be produced as a function of the type of defect (cavity inside the insulation, surface discharge in dirty or faulty insulations, corona effect in air in sharp-pointed conductive parts, etc.). If the measured pattern of the PDs in the entire cable is compared with the reference patterns, it is possible to observe whether there is a single defect or several defects. However, when there are several defects along the length of the cable, their corresponding patterns overlap at the measurement point and can be easily confused with one another, without it being easy to identify each and every one of the defects, the operator's experience being crucial for a correct diagnosis. Furthermore, the noise not removed in commercial techniques makes the identification of different sources of PDs through the simple observation of their patterns of PDs even more difficult.
According to the new invention, this problem is solved by automatically generating a pattern of PD pulses associated with the partial discharges located in each position along the length of the cable for the purpose of preventing the overlap of patterns associated with defects that were in different locations (different cable junctions, terminations) after the prior removal of the background noise. Once the patterns are separated by position, the invention includes an automatic defect pattern recognition tool, trained through a neural network so that the operator can emit an efficient evaluation of the insulation condition.